Andy Murray, fitter and more resilient, quicker and more tenacious than his opponent, had to stare defeat in the face and stare it down before winning the fourth Masters Series title of his career, the Canadian event, on a baking, muscle-aching, stamina-sapping day.

Murray's 6–7, 7–6, 6–1 win over Juan Martin Del Potro not only quelled the rising challenge of the youngest man in the world's top 10, but it showed immediately how important has been the benefit of his immense work in the even more intense heat in Miami before this tournament.

Within a minute of the finish of a 2hr 45min final which for much of the time Murray had looked the likely loser, he was thanking every member of the team that had helped him achieve what may well be the best physical condition of his career.

"It was difficult because Juan played unbelievably for the first two sets and I was on the defensive because of that, and I had to do a lot of running," Murray said. "But he has had a lot of matches in the last two weeks and I knew it would be difficult in this heat.

"But I needed to serve well because he was serving great. I hit three aces in the [second set] tie-break and that was the difference from the first set. Then I had to stay tough to stay on top."

Murray was referring to the fact the Del Potro had endured a long, hard tournament while winning the Washington title last week, as well as difficult contests in getting past Rafael Nadal and Andy Roddick here. But not all that Murray said was entirely so.

His first serve was for long stretches inconsistent, as it had been while securing the world No2 spot by beating Jo-Wilfried Tsonga on Saturday – though it did improve.

Murray also often chose the defensive role. He sought to take the pace off the ball, rolling it in, or slicing, with cat-and-mouse intentions, but sometimes hitting the ball solidly and then top-spinning in a slower one. Del Potro, who tried to force the issue much of the time, quite often looked susceptible to the Murray mixture and sometimes over-hit or drove into the net. But the Argentinian was also very dangerous. The forehand could do great damage even from the hinterland, and when he came to the net he was sometimes able, with the reach from his 6ft 6in frame, to smother any attempted passes.

Del Potro is also extremely fleet-footed for so large a man. After twice being within two points of losing the first set at 5–6, he motored from way outside the court to reach a Murray volley which had looked a complete formality, scraped it back, and somehow snatched the point.

When Del Potro followed it in the tie-breaker with an explosive serve, and a follow-up which secured him the set, the sixth seed had now become the favourite, and the mood of the match reflected that.

Nevertheless, Murray still chiselled out a chance to break for 5–4 and serve for the second set, beavering away from the back, and sometimes luring false strokes from Del Potro – only for the Argentinian to launch another forehand winner, and another ace, and to take the game with an improbable dinked winner, controlled while running full tilt.

Then the mood changed again. Del Potro annoyed Murray by making him wait to serve to save the set at 4–5, calling for an injury time-out for treatment to his left side. Murray, already back on court by that time, complained energetically to umpire Lars Graff and slung his towel away. It briefly brought back memories of how annoyed he had been 15 months ago in Rome when Del Potro had said of Murray that "he and his mother were always the same".

Certainly Murray looked fired up as he dropped only one point on his next two service games, and took the tie-break 7–3 thanks to a fine lob which helped him reach 5–3 and an ambushing volley which got him to 6–3.

Suddenly, for almost the first time in the match, Murray was on top. After more than two hours he was still moving well, plunging sideways to make a winning return of serve and reach break point, while Del Potro began to look sluggish, wafting a drive long to drop his first service game of the final set and slipping to 0–2 and 0–3.

When Del Potro lost his serve again, this time to love, and went 0–4 down, the match had been transformed in little more than a quarter of an hour.

So apparently has their relationship. At the end the tired Argentinian sauntered up and congratulated Murray with an enormous sweaty embrace. Unpleasant as that might have been, it seemed sincere.